1. Technical Field
This invention relates to a radio frequency (RF) front-end of a communication device and, in particular, to a system for varying the linearity of the RF front-end transmitter.
2. Related Art
In today""s society the presence and utilization of telecommunication systems is increasing at a rapid pace. Wireless and broadband systems and infrastructures continue to grow resulting in crossover between both types of systems. As such, wireless systems utilizing third generation xe2x80x9c3Gxe2x80x9d wireless standards (such as Code Division Multiple Access xe2x80x9cCDMAxe2x80x9d 2000 (a IS-95 based system), wideband CDMA xe2x80x9cWCDMA,xe2x80x9d frequency division duplex xe2x80x9cFDDxe2x80x9d and time division duplex approaches xe2x80x9cTDD,xe2x80x9d time Division-Spatial CDMA xe2x80x9cTD-SCDMAxe2x80x9d and others may be configured to handle a number of diverse applications other than voice transmission.
A few of these diverse applications may include low, medium and high data rate applications, such as, for example, e-mail, text, video and streaming applications. These communication devices may be configured to handle multiple modes of operation, including, without limitation, voice, low data rate, medium data rate, and high data rate modes of operation.
Each of these modes of operation may impose widely differing processing and/or transmitter requirements on the communication device. For example, the peak-to-average ratio, or crest factor, of the modulated baseband signal varies widely depending upon the nature of modulation, coding schemes, band-limited filtering employed, and the mode of operation. These different crest factors result in different linearity requirements for the RF front-end of a transmitter. The linearity requirements are important because of the stringent adjacent channel power/leakage specification requirements of many typical standards that should be met by the mobile transmitter. These specifications attempt to limit interference to other users in CDMA systems and thus increase the overall network capacity. To limit the adjacent channel interference to typically acceptable specification values, the transmitter front-end should be highly linear. This high linearity in turn determines the operating current for the RF front-end of the transmitter.
Unfortunately, energy is expensive and at times in short supply. Modern communication devices such as radios, televisions, stereos and computers consume a significant amount of power that translates into expensive electrical costs. Additionally, current mobile wireless devices (also known as mobile units) such as cellular telephones, portable televisions, portable radios, personal communication devices, pagers and satellites operate on battery power and thus have limited battery time. Limited battery time translates into limited continuous operation time.
In a multi-mode transmitter such as a transmitter configured to handle voice, low data rate, medium data rate, and high data rate applications, the conventional approach is to configure the transmitter for the application that imposes the most stringent linearity requirement on the system such as high data rate transmission. This approach results in excessive operating current in the other modes of operation. Because of the excessive operating current, the battery life, and hence talk-time, of the communication device will be decreased. Reduced talk-time in turn results in lower battery life and the necessity for frequent charging of the mobile communication device. Therefore, there is a need for a system that reduces the amount of power required by the RF front-end transmitter of a communication device.
This invention is a dynamically varying linearity system xe2x80x9cDVLSxe2x80x9d capable of varying the linearity of a radio frequency (RF) front-end of a communication device. As an example of operation, the DVLS would receive a condition signal indicating a desired mode of operation of a transmitter and the DVLS would adjust the linearity of the transmitter responsive to the mode of operation.
As an example implementation of the DVLS architecture, the DVLS may include a condition signal indicative of the desired mode of operation and a controller that adjusts the linearity of the transmitter responsive to the condition signal. The condition signal may be responsive to a user interface. The controller, responsive to the condition signal, may dynamically adjust the operating current of the transmitter. Additionally, the operating current of the transmitter may be set at a default level optimized for a certain mode of operation. The operating current of the transmitter may then be reduced from the default level for other modes of operation.
In one example implementation, the controller is capable of selecting a voice transmission mode, a low data-rate transmission mode, a medium data-rate transmission mode, and/or a high data-rate transmission mode. The selected mode, in combination with the desired transmit power, sets the operating current of the RF front-end of the transmitter. In another example implementation, a signal representative of the desired transmit power is provided externally, such as from a base station in communication with a mobile unit containing the transmitter, and exercising closed loop power control over the mobile unit. In this implementation, the base station exercises closed loop power control by first receiving a signal from the mobile unit, and then directing the mobile unit to either increase or decrease its transmission power in order to achieve a desired ratio of Eb/No. The mobile unit measures the received power from the base station and this value in turn determines the transmitted power of the mobile unit.
Other systems, methods, features and advantages of the invention will be or will become apparent to one with skill in the art upon examination of the following figures and detailed description. It is intended that all such additional systems, methods, features and advantages be included within this description, be within the scope of the invention, and be protected by the accompanying claims.